The present invention relates to the field of solenoid valves for controlling tool operation downhole in a hydrocarbon producing well and in subsea well installations. More particularly, the invention relates to a downhole solenoid valve constructed with a material having appropriate ferromagnetic and strength properties, and having high corrosion resistance to downhole well fluids and salt water.
Downhole solenoid actuated tools control the production of pressurized oil and gas in hydrocarbon producing wells and in subsea well applications. Solenoids control different operations including the opening and closing of valves, sliding sleeves, packers, wellheads, and other downhole well tools and subsea well systems. Solenoids and other tool actuators are typically constructed with ferromagnetic materials containing Iron (Fe) and Iron alloys. When a electric coil is actuated around the alloy, the Iron acts as a magnet for actuating the solenoid.
Although Iron alloys and stainless steel alloys conventionally provide the material for solenoid valves, such alloys are still corrosive and do not withstand exposure to downhole well fluids and salt water found in subsea installations. Many efforts have been made to improve solenoid valve corrosion resistance while retaining the ferromagnetic properties of the valve. In a pure form, Iron is relatively weak and requires alloy additions to increase strength and corrosion resistance. Iron is typically alloyed with carbon (C) and with Silicon (Si) to increase strength. Neither of these elements increase corrosion resistance, accordingly small amounts of Chromium (Cr), Molybdenum (Mo), and Manganese (Mn) are added to Iron alloys to increase the corrosion resistance without significantly reducing the ferromagnetic properties of the solenoid valve material. For example, U.S. Pat. No. 4,770,723 to Sagawa et al. (1988) disclosed a substantially Iron magnet including rare Earth elements and intermetallic compounds where at least 50% of the entire material had a Fe-B-R type tetragonal structure,
Another technique for reducing corrosion was shown in U.S. Pat. No. 5,529,747 to Learman (1996), where up to 35% of an epoxy binder was mixed with a ferromagnetic material such as Iron powder to isolate the Iron particles from corrosion. Other sintered magnets using Cobalt (Co) in a permanent magnet were disclosed in U.S. Pat. No. 3,892,598 to Martin (1975) and in U.S. Pat. No. 4,533,407 to Das et al. (1985).
Various alloys have been developed to improve magnet performance. U.S. Pat. No. 4,404,028 to Panchanathan et al. (1983) disclosed a nickel rich alloy having Copper and small amounts of Boron to produce improved mechanical properties and good corrosion and oxidation resistance. Other alloys have been developed to provide strength to materials which also provide ferromagnetic properties, such as in U.S. Pat. No. 4,297,135 to Giessen et al. (1981) which disclosed a high strength Iron, Nickel and Cobalt base crystalline alloy having dispersion of borides and carbides. The alloy contained Iron, Nickel or Cobalt in a range between 85-95% by atomic percentages, at least 3% Boron, and the balance from a select metal group. U.S. Pat. No. 4,133,680 to Babaskin et al. (1979) disclosed dopant materials in weight amounts ranging between 3 to 25 by volume percent for combination with a base of Iron and Nickel.
Other alloys have been developed to increase wear characteristics while maintaining superb magnetic properties for magnetic recording and reproduction. In U.S. Pat. Nos. 5,725,687 (1998) and 5,496,419 (1996) to Murakami et al., wear resistant, high permeability alloys having Nickel and Iron in combination with other elements including less than 3% of Beryllium (Be), Silver (Ag), Strontium (Sr), and Barium (Ba). However, these alloys were developed for the recording industry where wear characteristics are important and yield strength is relatively unimportant.
In addition to corrosion considerations, the narrow confines of downhole wellbores limit the space available for solenoid valves and other solenoid actuated equipment. Larger solenoid valves cannot easily be placed transversely in the wellbore, and larger solenoid valves occupy space interfering with the production of hydrocarbon fluids. In deep wells, high fluid pressures require a solenoid having sufficient ferromagnetic properties to actuate downhole equipment. There are no known materials or compounds which combine the requisite corrosion resistance, mechanical strength, and ferromagnetic properties to adequately provide a small solenoid valve capable of operating downhole in a wellbore.
Various alloys have been developed to provide corrosion resistant solenoid valves. Necessary properties for the alloys comprise a high saturation induction to develop a strong magnetic field for reducing the actuation energy required, high permeability for permitting the development of small, efficient components, a low coercive field strength permitting rapid magnetization and demagnetization for fast valve operation, freedom from magnetic aging so that the magnetic properties are sustained over time, electrical resistivity for efficient operation of solenoid valves, and corrosion resistance for withstanding downhole corrosive fluids.
Silicon is added to low Carbon Iron to increase hardness and electrical resistivity, however such alloys have minimal resistance to corrosive environments and are often plated to build corrosion resistivity. Chromium-Iron alloys provide good corrosion resistance and adequate magnetic properties for core applications, however such alloys allow higher core losses and provide lower saturation and permeability than Silicon-Iron alloys. An example of a Chromium-Iron alloy is Type 430F solenoid quality stainless steel, having 18% Chromium content and small quantities of Molybdenum, which has superior magnetic properties and low residual magnetism when compared to other stainless steels.
Other solenoid alloys known as Chrome-core alloys are controlled-chemistry, ferritic, Chromium-Iron alloys having superior corrosion resistance to pure Iron, low-Carbon steel, or Silicon-Iron alloys, yet having greater immunity to the saturation induction decline associated with 18% Chromium ferritic stainless steels. Various of the Chrome-core alloys have 8% and 12% Chromium, and have flux densities approaching Electrical Iron and Silicon Core Iron at magnetic field strengths exceeding 800 A/M. 13% Chromium alloys further raise the electrical resistivity while providing good corrosion resistance and stable ferrite, and Molybdenum and Niobium have been added to 18% Chromium-core alloys to increase corrosion resistant properties while providing relatively high electrical resistivity.
One commercially available solenoid alloy marketed as Hiperco 50A Alloy by the Carpenter Technology Corporation of Reading, Penn. incorporates 0.01% Carbon, 0.05% Manganese, 0.05% Silicon, 48.75% Cobalt, 1.90% Vanadium, and the balance Iron. This material is used primarily as the magnetic core material in electrical equipment requiring high permeability at very high magnetic flux densities, and has electrical resistivity of 253 ohms c/mf and 420 microhm-mm. The relatively high Iron content of this alloy limits the use of this solenoid alloy in high corrosion applications.
There is, accordingly, a need for an improved material for providing strength, corrosion resistance, and magnetic performance for use in downhole well applications and in subsea well systems.